It's official: It's hard to lose weight

BAD news for people trying to lose weight. A new assessment of how the body responds to dieting shows that it is about twice as difficult as previously thought to shed fat.

The calculations also puncture the myth that cutting calories will lead to continued weight loss. In fact, the effect of reducing the intake of calories levels off after about three years, scientists said.

Obesity experts had previously suggested that cutting 100 kilocalories a day from the diet for six months would lead to the loss of about 5lb in weight. But it turns out that this amount of weight loss is more likely to take a year and that any further weight loss will reach a plateau after three years, rather the continuing forever at the same rate.

"People have used this rule of thumb for how to lose weight for decades and it turns out to be completely wrong," said Kevin Hall, a mathematical modeler at the US National Institute of Diabetes and Kidney Diseases in Maryland.

"The reason it's wrong is that it doesn't account for the metabolic changes that take place when people change their diet. If you cut the calories in someone's diet, the metabolism slows down and it slows down the more weight that is lost until it reaches a plateau."

A popular misconception is that overweight or obese people have a slower metabolism, which would mean that they do not burn off the calories as fast as leaner people. In fact, the fatter someone is, the higher their metabolism is likely to be be, Dr Hall told the American Association for the Advancement of Science.

"Unlike the popular notion out there that heavier people have slower metabolism, the heavier you are the more calories you are burning," he said. "And the more of that mass is lean tissue compared to fat tissue, the more calories still you are burning - and obese people tend to have both an elevated fat mass and an elevated lean-tissue mass."

American scientists have used the new assessment to build a mathematical model of how the human body responds to variations in the intake of food energy.

"Using the old system, someone who cut 100 calories a day could expect to lose 5lb in six months," Dr Hall said. "Under the new system, it would take a year to lose this 5lb - making it twice as difficult. The rule of thumb is cutting 10 kilocalories per day from your diet for every pound you want to lose."

Dr Hall said that giving people unrealistic expectations of how easy weight loss is can itself be dispiriting if the advice turns out to be wrong.